The present invention relates to an alkaline storage battery, a nickel positive electrode plate for use in an alkaline storage battery and the methods for producing the same.
As an environmentally friendly battery exhibiting high energy density, high output and the like, the sealed nickel-metal hydride storage battery has been widely used for the power sources of cordless equipment such as communications equipment and personal computers, and electronic equipment. The nickel-metal hydride storage battery has also been applied to power tools, electric vehicles and the like, each of which requires large current charge and discharge.
A nickel-metal hydride storage battery is obtained by laminating a nickel positive electrode containing nickel hydroxide as the active material and a negative electrode comprising a hydrogen storage alloy containing hydrogen as the active material, with an alkali-proof separator interposed therebetween, and impregnating the laminated body thus obtained with an approximately 7 to 8 N alkaline electrolyte, followed by sealing.
Such a nickel-metal hydride storage battery generally has a drawback of having a relatively high self-discharge rate as compared with a nickel-cadmium storage battery. One factor in accelerating the decomposition rate of the positive electrode active material is believed to lie in that hydrogen reduces the positive electrode active material. In the case of a nickel-metal hydride storage battery using a negative electrode comprising a hydrogen storage alloy, the partial pressure of hydrogen is always present in the battery, inducing a self-discharge reaction in which hydrogen reduces the positive electrode active material to discharge the same.
Moreover, nickel hydroxide as the active material has a relatively low electronic conductivity. For this reason, in order to suppress a decrease in working voltage during discharging, while maintaining a high utilization of the positive electrode active material at a high temperature, cobalt hydroxide, which is a highly conductive substance, is added in the positive electrode. In a nickel-metal hydride storage battery configured by using such a positive electrode, cobalt hydroxide in the positive electrode is charged during charging to be converted into xcex2-CoOOH, which functions as a conductive agent. This xcex2-CoOOH is stable because it does not change during a normal charging and discharging and has a low solubility. It is, however, converted into cobalt hydroxide having a high solubility, when the battery is allowed to stand for a long period without applying any load thereto until the battery voltage reaches approximately 1.0 V or less. Additionally, when the battery is used for a power tool, electric vehicle and the like, a large current discharge is carried out, so that the positive electrode is partially overdischarged, converting the xcex2-CoOOH into cobalt hydroxide. The reduced cobalt, particularly the cobalt on the surface of the positive electrode is caused to migrate to the separator. When the battery is recharged in such a state, a cobalt ion in the separator that is in contact with the surfaces of the positive and negative electrodes is oxidized to form a minute conductive network between the positive and negative electrodes (hereinafter referred to as xe2x80x9ca minute chemical short circuitxe2x80x9d), resulting in a problem of inducing the self-discharge of the battery.
With the well-known modifications as described above, the self-discharge of the nickel-metal hydride storage battery can be suppressed to some extent; however, such effect is insufficient, and no effective method has been found for specifically preventing the formation of the minute chemical short circuit.
Meanwhile, the following methods for suppressing the self-discharge have hitherto been proposed: a sulfonation treatment of the separator (Japanese Unexamined Patent Publication No. sho 62-115657); a sulfonation treatment of the surface of the negative electrode (Japanese Unexamined Patent Publication No. hei 8-315810); the addition of a manganese compound in the positive electrode (Japanese Unexamined Patent Publication No. hei 5-121073); and the like. In addition, for suppressing the deterioration of the battery capacity both after storage and standing for a long period, it has been proposed to place inside the battery, a microcapsule obtained by sealing a manganese compound having a higher oxidizing ability than cobalt oxyhydroxide in a polymeric compound (Japanese Unexamined Patent Publication No. hei 8-255628).
Although it has been propose to use a separator obtained by sulfonating an olefin type resin, this poses a problem that the sulfonation treatment reduces the strength of the separator to induce a physical short circuit between the positive and negative electrodes more easily, thereby possibly shortening the charge/discharge cycle life.
Also, when adding a manganese compound in the positive electrode, the surfaces of the positive electrode active material and the current collector are coated with the manganese compound, resulting in the problem that the charge efficiency and the large current charge/discharge characteristic of the battery are decreased. Furthermore, when using a microcapsule obtained by sealing a manganese compound having a higher oxidizing ability than cobalt oxyhydroxide in a polymeric compound, a high temperature is required in order to destroy the microcapsule, so that the negative electrode alloy becomes susceptible to oxidation thereby to deteriorate the performance of the negative electrode. In addition, after destroying the microcapsule, a manganese ion having a high oxidizing ability, which has been diffused in the negative electrode, oxidizes the alloy, thereby deteriorating the charge/discharge performance of the negative electrode.
The present invention is to solve such a problem and is aimed at providing a nickel-metal hydride storage battery exhibiting an excellent self-discharge resistance, while preventing the formation of a minute chemical short circuit between the positive and negative electrodes of the battery.
A nickel positive electrode plate in accordance with the present invention includes a porous nickel substrate and an active material comprising a hydroxide of nickel filled into the substrate, the positive electrode plate having a layer of a manganese compound containing manganese with a valence of 2 or more on the surface thereof.
It is effective that the active material is a solid solution of a hydroxide of nickel containing at least one selected from the group consisting of cobalt, zinc, magnesium and manganese.
It is also effective that the active material has a hydroxide of cobalt on the surface thereof.
It is also effective that the layer of a manganese compound has a thickness of 0.1 to 20 xcexcm.
Further, an alkaline storage battery in accordance with the present invention comprises: the above nickel positive electrode plate; a negative electrode plate; a separator; and an alkaline electrolyte.
Then, a method of producing a nickel positive electrode plate in accordance with the present invention comprises the steps of: (1) filling an active material comprising a hydroxide of nickel into a porous nickel substrate; and (2) forming a layer of a manganese compound on the surface of the substrate filled with an active material.
In this production method, it is effective that the step (2) is a step of forming a layer of a manganese compound on the surface of the substrate by charging and discharging the substrate filled with an active material at least once, and immersing the substrate in a saturated alkaline solution containing manganese ions.
Alternatively, it is effective that the step (2) is a step of forming a layer of a manganese compound on the surface of the substrate by immersing the substrate filled with an active material in a saturated alkaline solution containing manganese ions, while applying a potential to the substrate.
Alternatively, it is effective that the step (2) is a step of forming a layer of a manganese compound on the surface of the substrate by applying a manganese compound containing manganese with a valence of 2 or more onto the surface of the substrate filled with an active material, and immersing the substrate in a saturated alkaline solution containing manganese ions, while applying a potential to the substrate.
Further, a method of producing an alkaline storage battery in accordance with the present invention comprises the steps of: (1) causing any one of a positive electrode plate, a negative electrode plate and a separator to retain a powder of metallic manganese or a manganese compound containing manganese with a valence of 2 or more; (2) assembling a battery by using the positive electrode plate, the negative electrode plate, the separator and an alkaline electrolyte; and (3) charging and discharging a resultant battery at least once.
In this case, it is effective that the step (1) is a step of applying a manganese compound containing manganese with a valence of 2 or more onto the surface of the positive electrode plate.
Alternatively, it is effective that the step (1) is a step of applying a manganese compound containing manganese with a valence of 2 or more onto the surface of the separator.
Alternatively, it is effective that the step (1) is a step of applying a manganese compound containing manganese with a valence of 2 or more onto the surface of the negative electrode plate.
It is effective that said step (1) is a step of adding a powder of metallic manganese or a manganese compound containing manganese with a valence of 2 or more in the negative electrode plate.
Alternatively, it is effective that the step (1) comprises a step of forming manganese hydroxide in a pore of the separator by immersing the separator in an aqueous solution of a manganese salt, followed by immersing in an aqueous alkaline solution, and another step of drying a resultant separator containing manganese hydroxide in an inert atmosphere or under a reduced pressure.